This invention relates to a semiconductor substrate having an improved metallization system disposed thereon, to semiconductor packages comprising semiconductor devices having the improved metallization system of this invention and to a method for making such semiconductor packages.
Semiconductors generally consist of a single crystal element of silicon which has an internal PN junction and has plated metal contacts at surfaces parallel to the PN junction. Electrical connections are made to the metal contacts and often the connections are part of the housing which forms an enclosure for the semiconductor device. Metal coatings have been required on the semiconductor devices for various reasons. Thus, in those cases where electrical connectors are soldered to the device, the solder materials do not wet silicon well enough to permit soldering the connectors directly to the silicon material. In packages where contacts are made directly to conductive studs, the metal coatings are required to provide an electrical contact by pressure alone.
Various metallization systems are used in the semiconductor industry but these known systems are characterized by a variety of drawbacks. For example, one widely used system is the chromium-silver-gold metallization used on numerous devices. This system suffers from reproducibility problems when evaporated onto silicon substrates by conventional evaporation equipment. Furthermore, at high temperatures, e.g., 700.degree. C. or above, gold migrates into silicon therefore requiring a thick layer of chromimum as a barrier. Double stud whiskerless diode packages have become common in recent years. In these packages, pressure contact is made between the conductive studs and the device inside an impervious housing. However, the high temperatures necessary to seal the package create problems because of the unmatched coefficients of thermal expansion of the components of the package. Thus, the studs pull apart from the diode in operation resulting in high temperature opens and device failure.